1. Technical Field
The present disclosure relates to using a variable camshaft timing device to deactivate a portion of engine cylinders.
2. Background Art
Variable camshaft timing (VCT) devices are used to change the phase relationship between the crankshaft and the camshaft lobes that control actuation of the intake and exhaust poppet valves. By controlling the phase relationship, the fuel economy, performance, and emissions can be improved by providing less valve overlap at low engine speed operation to improve combustion stability, and more valve overlap at higher engine speed to improve engine power and performance.
It is known that by deactivating a portion of engine cylinders when the engine is operating at light load, that overall fuel economy can be improved. Typically, cylinders are deactivated by deactivating intake and exhaust poppet valves. In U.S. Pat. No. 6,237,559 B1, commonly assigned to the assignee of the present application, cylinder deactivation is provided by disabling exhaust poppet valves. The intake valves are not disabled, but are opened and closed generally symmetrically about a top center or bottom center position of the piston. This results in a cost savings because valve deactivators are provided for exhaust valves and not intake valves. However, such a system does not eliminate valve deactivators, which are known to be problematic. For example, there are typically issues associated with latching the valve deactivators and difficulties in ensuring that they are latched within one engine cycle, particularly at high engine speeds.
In U.S. Pat. No. 5,642,703 commonly assigned to the assignee of the present application, cylinder deactivation is provided by adjusting a single camshaft which actuates both the intake and the exhaust valves, or alternatively, using dual camshafts. However, this reference teaches that both camshafts may be linked together with one phase and that adjusting the timing of the valve lift events has no effect on the relative timing between the exhaust valve lift event and intake valve lift event, i.e., the timing between exhaust and intake valve lift events remains constant regardless of phase shifting. Because the intakes and exhausts are not independently adjusted, one embodiment shows both valve events retarded such that the exhaust valve event is roughly centered about top center (TC) of piston movement between the exhaust and intake strokes and the intake valve event is roughly centered about bottom center (BC) between the intake and compression strokes. The valve events are optimized for normal operation, not for deactivated operation. Thus, it is not possible to adjust both intake and exhaust valve events such that they are symmetrical about a rotational position of the crankshaft when the direction of the piston changes, i.e., TC or BC. Also, valve overlap, which lasts about 25 degrees of crank rotation is not adjusted in such a system because the intake and exhaust valve events move in lock step. It is typical for the intake system to operate at a vacuum and the exhaust system to be at a pressure slightly higher than atmospheric. Thus, during the valve overlap period while the valves are in the deactivated condition, exhaust gases flow into the intake and are then inducted by the activated cylinders. While there may be operating conditions where such flow from exhaust to intake is advantageous, it is desirable to control such flow. For example, when operating at low torque conditions, there is a considerable intake manifold vacuum to drive flow from the exhaust to the intake. However, this is an operating condition where no additional exhaust gases can be accommodated without impairing combustion stability. At higher torque conditions, exhaust gases can be accommodated in the combustion charge, but there is little manifold vacuum to drive the flow from exhaust to intake. While the strategy for valve deactivation disclosed in U.S. Pat. No. 5,642,703 is suitable for many applications, various advantages may be provided by appropriate control of valve overlap that cannot be achieved with such a strategy.